1. Field of the Invention
The present invention relates to an ion implantation apparatus and an ion implantation method, and more specifically, it relates to an ion implantation apparatus for ionizing gas and implanting the ions as formed into a wafer which is placed in the apparatus, and an ion implantation method.
The present invention also relates to a semiconductor device, and more specifically, it relates to a semiconductor device which is obtained by implanting ions into a wafer.
2. Description of the Background Art
A conventional ion implantation apparatus and a conventional ion implantation method are now described.
FIG. 8 is a typical diagram showing the structure of a conventional ion implantation apparatus. FIG. 9 is a flow chart showing steps of a conventional ion implantation method.
Referring to FIG. 8, the conventional ion implantation apparatus has an ion source, lead electrodes 209a and 209b, a mass spectrograph 211, an accelerator 213, and a vacuum chamber 215.
The ion source has an ion generator vessel 205 and a pair of electrodes 201a and 201b. The pair of electrodes 201a and 201b are arranged in the ion generator vessel 205 to be opposed to each other. A power source 201c applies a potential difference across the pair of electrodes 201a and 201b.
The ion generator vessel 205 is provided with a gas inlet path 205a for introducing reactive gas into the interior, and a gas outlet path 205b for discharging the reactive gas from the interior.
The lead electrodes 209a and 209b are adapted to draw ions which are generated in the ion generator vessel 205. A power source 209c applies a potential difference across the lead electrodes 209a and 209b. The mass spectrograph 211 is adapted to select target ions from those drawn by the lead electrodes 209a and 209b by changing the intensity of a magnetic field. The accelerator 213 is adapted to accelerate the ions for introducing the same into the vacuum chamber 215 at a prescribed speed. A wafer 20 is placed in the vacuum chamber 215, so that the ions are implanted into the same.
The ion implantation method which is carried out in the aforementioned ion implantation apparatus is now described.
Referring to FIGS. 8 and 9, gas or steam is first introduced into the ion generator vessel 205 through the gas inlet path 205a (step 231). The power source 201c applies a potential difference across the electrodes 201a and 201b (step 232). Thus, arch discharge is caused to ionize the gas (step 233).
The power source 209c applies a potential difference across the lead electrodes 209a and 209b, whereby the ions are drawn from the ion generator vessel 205 along arrow (step 234). The mass spectrograph 211 selects only target ions from the drawn ones (step 235). The accelerator 213 accelerates the target ions to the prescribed speed (step 236). Thus, the target ions are accelerated to the prescribed speed along a path R, and thereafter implanted into the wafer 20 which is placed in the vacuum chamber 215 (step 237).
Thus, the ions can be implanted into the wafer 20 by the conventional ion implantation apparatus and the conventional ion implantation method.
However, the conventional ion implantation apparatus and the conventional ion implantation method described above have the following problems (13) to (13), as hereafter described in detail.
(1) A p-type semiconductor substrate may be provided with a p-type retrograded well, in order to prevent a leakage current. This retrograded well is generally formed by ion-implanting boron (B) into the substrate. The boron B includes .sup.10 B and .sup.11 B, which are isotopes having mass numbers of 10 and 11 respectively, as shown in the following Table:
__________________________________________________________________________ Implantation in the Same Depth Abundance Implantation Energy Merit/Demerit __________________________________________________________________________ .sup.11 B 81% 1 MeV Throughput: About 3 min. per Wafer (Batch) .sup.10 B 19% Reduced by 9% to 910 keV Damage and Metal Contamination Reduced by 9% Throughput: About 12 min. per Wafer (Batch) __________________________________________________________________________ *Throughput: at the same gas flow rate
Here, the time of throughput depends on the amount of implantation, and, in the Table above, the throughput shown is of .sup.10 B implantation when throughput of .sup.11 B implantation is 3 minutes.
When .sup.10 B which is lighter than .sup.11 B is employed, the implantation energy can be reduced by about 9% as compared with a case of ion-implanting .sup.11 B into a semiconductor substrate. In such ion implantation of .sup.10 B, therefore, it is possible to reduce damage by about 9% to the semiconductor substrate and metal contamination as compared with the ion implantation of .sup.11 B.
However, the natural abundance of .sup.10 B is 19%, i.e., about 1/4 of that of .sup.11 B (81%). Thus, raw material gas of BF.sub.3 which is employed in boron implantation generally contains .sup.10 B and .sup.11 B in the aforementioned natural abundance ratio. In ion implantation through the conventional ion implantation apparatus and the conventional ion implantation method employing the raw material gas of BF.sub.3, therefore, the beam current value of .sup.10 B is about 1/4 of that of .sup.11 B, disadvantageously leading to reduction of the throughput. In other words, it is impossible for the conventional ion implantation apparatus and the conventional ion implantation method to improve the throughput while suppressing damage to the semiconductor substrate.
In general, further, there has been obtained no semiconductor device having a boron-implanted region such as a retrograded well, for example, which is inhibited from damage caused by ion implantation.
(2) In the conventional ion implantation apparatus and the conventional ion implantation method, the gas is ionized by arc discharge. In this case, all molecules and atoms forming the gas components are ionized. In other words, not only the target component but the remaining components are ionized. Therefore, excess energy is required for the ionization as compared with the case of ionizing only the target component, leading to remarkable energy loss.
(3) In the conventional ion implantation apparatus and the conventional ion implantation method, the electrodes 201a and 201b must be arranged in the ion generator vessel 205, in order to ionize the gas by arc discharge. These electrodes 201a and 201b are generally made of carbon. Therefore, the carbon inevitably adheres to inner walls of the ion generator vessel 205 etc. by sputtering in the arc discharge. Thus, it is necessary to remove the adhering carbon, with requirement for much labor for maintenance.
(4) In addition to the electrodes 201a and 201b, the lead electrodes 209a and 209b for drawing the ions are also arranged in the ion generator vessel 205. Thus, the ion generator vessel 205 is provided therein with a number of electrodes, which restrain each other. Thus, it is difficult to set conditions for and arrangement of the electrodes.
(5) In the conventional ion implantation apparatus and the conventional ion implantation method, all molecules and atoms of the gas are ionized by the arc discharge. Thus, a plurality of species of ions may be drawn by the lead electrodes 209a and 209b. The mass spectrograph 211 is adapted to select only the target ions in this case. In other words, the mass spectrograph 211 implants only the target ions into the wafer 20 along the path R, while the remaining ions are made to collide with inner walls S.sub.1 and S.sub.2 of the apparatus along paths R.sub.1, and R.sub.2. Sputtering is caused by such collision of the ions with the inner walls S.sub.1 and S.sub.2, resulting in contamination etc. Due to such contamination, a bad influence such as leakage is exerted on a device which is formed on the semiconductor substrate.
(6) As hereinabove described, the conventional ion implantation apparatus and the conventional ion implantation method inevitably require the mass spectrograph 211 for selecting the ions since a plurality of species of ions may be drawn by the lead electrodes 209a and 209b. Thus, the apparatus is complicated in structure and increased in size due to such provision of the mass spectrograph 211.